This invention relates, generally, to a journal air bearing (hydrodynamic fluid film journal bearing or foil bearing) and, more specifically, to such a bearing used in AMS rotating machinery.
As known, a journal air bearing supports a rotatable component, such as a shaft disposed within an air-cycle machine of an aircraft. A typical arrangement of such a bearing includes a top foil, an intermediate foil, and a bump foil. The foils are received within a journal sleeve and wrapped around the component. The top foil is disposed closer to the component than is each of the other foils. The bearing uses a fluid, such as air, to support the component during its rotation. There is little or no contact between the top foil and component when it rotates.
The bearing may include also a formed key that extends radially relative to an axis of rotation of the component. The key is received within a slot or keyway defined in the sleeve and contacts edges defined by the slot to limit rotation of the foils relative to the component. The air communicates through a clearance provided at the key and is pressurized during the rotation. The bearing acts as a pump that draws in ambient air axially along the keyway and then through viscous-shear drives that drive the air circumferentially around the bearing, creating a high-pressure pocket or air film to react to radial loads. The air film defines an entrance or a supply area. The pressure varies depending upon a circumferential location thereof relative to the keyway, such varied pressure being able to destabilize the component. The pressure can typically peak at about 180° from the key.
It is desirable to provide increased capacity to the bearing through an increased supply area to the air film, which results in lower entrance losses and a more efficient pumping action for viscous shear that produces the air film.